1. Field of the Invention
This present invention relates to a heat roller type fixing device for use in an electrophotographic copier, printer, facsimile apparatus or similar electrophotographic image recorder.
2. Discussion of the Background
A heat roller type fixing device is extensively used with an electrostatic image recorder and is constituted by a pair of coactive rollers, i.e. a fixing roller and a pressing roller. The fixing roller is made up of a hollow tubular metal core and a heating element disposed in the metal core. Held in pressing contact with the fixing roller, the pressing roller is comprised of a hollow tubular metal core and a heat-resisting rubber layer provided on the outer periphery of the metal core. In the case that the fixing and pressing rollers have a substantial length for accommodating paper sheets, the width and therefore the size of which is relatively large, the metal core of each roller is usually provided with a thin wall to reduce the weight of the roller, thermal capacity, ect. However, a problem with a fixing device having such long fixing and pressing rollers is that when the pressing roller is pressed against the fixing roller at opposite ends thereof, it is deformed by the pressing force because the span between bearings which are provided at opposite ends of the rollers is long. This effects distribution of nip width defined between the fixing and pressing roller in the lengthwise direction of the rollers, i.e., the nip width is greater at opposite end portions than at the intermediate portion. Consequently, the amount of heat applied to a paper sheet is smaller at the intermediate portion of the rollers than at the opposite end portions, resulting in irregular fixation. Further, the force available for transporting a paper sheet differs from the intermediate portion of the rollers to the opposite end portions, often causing the paper sheet to crease. Although such problems may be solved by increasing the thickness and therefore the rigidity of the walls of the metal cores of the rollers, this kind of implementation brings about another problem in that the mass of each metal core is increased to in turn increase the period of time necessary for the fixing device to reach an operable temperature, i.e. buildup time.
In the light of the above, there has been proposed a fixing device in which the pressing roller is provided with a hollow hand-drum configuration i.e., its diameter is slightly increased at opposite end portions rather than at an intermediate portion, as disclosed in Japanse Laid-Open Patent Publication (Kokai) No. 58-154962 by way of example. A pressing shaft is disposed in the hand-drum type pressing roller while a single bearing is mounted on an intermediate portion of the pressing shaft for supporting the pressing roller. Pressing forces applied to opposite ends of the shaft are imparted to the pressing roller by way of the shaft and bearing, whereby the two rollers are pressed against each other under a substantially uniform force distribution throughout the length of the rollers. This kind of device, however, encounters a problem when the pressing roller has a substantial length and undergoes oscillations due to limited machining accuracy. Specifically, the oscillations cause the contact pressure to increase at the intermediate portion with the result that the nip width becomes greater at the intermediate portion than at opposite end portions. Further, the sheet transport speed is increased at the intermediate portion and not at the opposite end portions, causing a paper sheet to crease. Another drawback with this scheme is that providing the pressing roller with an accurate hand-drum configuration is disproportionately time- and labor-consuming.